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Appendix I.

7S5

as shewn at p. 329, and the work to be welded consisted of
general machinery repairs (especially for-engine breakdowns) and
boiler repairs. In addition the arc was used for boring holes in
plates, or for otherwise melting portions of metal away. The
process is of great advantage for breakdowns, as putting the
machinery in working order in an extremely short space of time,
and is also especially useful for patching purposes, thus saving
many articles from the scrap heap. The average ratio of weld to
solid was 85-5 per cent, for iron, and 80*8 for steel, as shewn by
testing. The best hand welds were found to be much below this.

P. 693. Fusible Plugs made of gun-metal are screwed into
the crown of boiler fire-boxes or furnaces. They are drilled
through the centre and filled with a fusible metal whose com-
position depends on the temperature at which it is desired the
steam shall blow out the fire when shortness of water occurs.
The following table will shew the composition required:

Melting Temperature
in degrees F.

201
2O2
202
246

257
266

334
334
392

Composition of Metal in parts

by Weight.

Lead.           Tin.         Bismuth.

I                I                4

5

2
I
I
I
O
I

Tin.
I

3
3
4
o
i

2

3

5
5
i
o
i
o

~                o

(See also App. //., p. 833.)

CHAPTER VIII.
P. 363-6. Nature of Shear Stress.

Fc - ,/lFg   and   Ft 

But Fc and Ft each act on areas of */2 while F8 acts on an
area = i (see Mg. 324):